Prevalence and severity of disease of the protozoan parasite Toxoplasma gondii varies geographically in the wide array of hosts T. gondii infects. These facts highlight the need to discover the transmission dynamics and the genetic relationship among strains that are causing the wide spectrum of disease states identified in nature. Specifically, our research addresses the emergence and re-emergence of parasitic zoonotic diseases that have complex life-cycles with multiple routes of transmission that impact people and animals who co-exist in the same ecological niche. We seek discoveries in these areas to support the development of new diagnostic tools, discover fundamental paradigms governing virulence shifts in parasitic protozoa and ultimately develop efficacious anti-protozoal strategies to mitigate the spread of disease. This year we identified a remarkable diversity of Toxoplasma strains infecting, and causing die-offs, in native Australian marsupials. Of 46 marsupials screened for T. gondii by multilocus PCR-DNA sequencing at polymorphic genes (B1, SAG3, GRA6, GRA7), 12 were PCR-positive;the majority (67%;9/12) were infected by nonarchetypal Type II-like or atypical strains, indicating greater diversity of genotypes than previously envisaged. Two isolates lethal to marsupials, were avirulent to mice, and screening of the human population showed that strains infecting the marsupials are also infecting people. The data support the conclusion that Australias isolation may have favored the persistence of nonarchetypal ancestral genotypes that have likewise infected people. In another study, we used multilocus DNA sequencing to identify another nonarchetypal strain of Toxoplasma gondii as the causal agent of a 2001 waterborne outbreak in Brazil. The strain, isolated from a water supply linked epidemiologically to the outbreak, was virulent to mice, and has previously been identified as BrI. Using a serologic assay that detects strain-specific antibodies, we found that the majority (65%) of individuals who were IgM-positive during the outbreak possessed the same serotype as mice infected with the purported epidemic strain. The remaining individuals, plus additional IgM-negative, IgG-positive individuals not infected during the outbreak, possessed one of four novel serotypes, the most common of which matched the serotype of mice infected with strains isolated from chickens foraging near the outbreak site. The latter strains likely reflect the genetic diversity of T. gondii circulating in highly endemic regions of Brazil. The strain-typing assay identified those specific individuals infected with the outbreak agent, and proved a useful epidemiologic tool for future outbreaks that occur in areas having a large genetic diversity of parasites. Both studies also implicated the cat as an important vector for promoting transmission of Toxoplasma gondii and the oocyst as the pivotal life cycle stage for epidemic expansion of parasite clones in nature. These data provide important data from a public health perspective because they highlight the relevance of the oocyst stage of the parasite (shed by the definitive host) in naturally occurring disease and helps focus prevention strategies, such as filtering water or definitive host population control, accordingly.